Method and apparatus for determining filtrability time



Juiy 15, M69 F. T. SMITH ETAL METHOD AND APPARATUS FOR DETERMINING FILTRABILITY TIME 2 Sheets-Sheet 1 Filed June 6, 1967 INVENTOR FIIQA A 7'. SM/ 771 RUBERT 0. 3mm BY 7%fl 2m July 15, 1969 "r, s n'H T 3,455,146

METHOD AND APPARATUS FOR DETERMINING FILTRABILITY TIME Filed June 6. 1967 2 Sheets-Sheet :3

T INVENTOR flaw/408 5 3,455,146 METHOD AND APPARATUS FUR DETERMINING FILTRABHLHTY TIME Frank T. Smith and Robert D. Smith, Wilmington, DeL, assignors to E. I. du Pont de Nemours and Company,

Wilmington, DeL, a corporation of Delaware Filed June 6, 1967, Ser. No. 643,930

lint. Cl. Gtlln 15/04 U.S. Cl. 73-61.4 Claims ABSTRACT OF THE DISCLOSURE A method for determining the time in which a device will develop a given pressure drop or flow rate when subjected to a process stream containing a substantial amount of material that does not pass through the device, which comprises:

(a) Determining the equivalent number of openings of the device by comparing the device and a strainer when each is subjected to the same test stream, the strainer having a mesh size such that it removes only such size material as would be entrapped by the device, and

(b) Thereafter, passing the process stream through the same strainer and correlating the time dependent pressure drop across or flow rate through the strainer to the time in which the device will develop a given pres sure drop or flow rate based on the equivalent number of openings of the device.

Under constant flow conditions, the pressure drop across the strainer as a function of time may be continuously determined by reversing the strainer with respect to the process stream flow or vice versa whenever the pressure drop reaches a predetermined value.

Background of the invention Fouling of apparatus (e.g., heat exchangers, filter beds, etc.) by particles, sludge, debris, slime and other entrained matter hampers the eflicient operation of many industrial processes. It is extremely desirable, therefore, to predetermine the time such apparatus can function before a maximum pressure drop and/ or minimum flow rate is reached. This determination provides an important factor in considering the type and size of equipment employed, and the feasibility of periodic cleaning and/or installation of solids removal equipment.

Summary of the invention The present invention is a method for determining the time in which a device will develop a given pressure drop or flow rate when subjected to a process stream containing a substantial amount of material that does not pass through said device, which comprises:

(a) Determining the equivalent number of openings of said device by comparing said device and a strainer when each is subjected to the same test stream, said strainer having a mesh size such that it removes only such size material as would be entrapped by said device, and

(b) Thereafter, passing the process stream through said strainer and correlating the time dependent pressure drop across or flow rate through the strainer to the time in which said device will develop a given pressure drop or flow rate based on the equivalent number of openings of said device.

One apparatus which may be used to continuously determine the pressure drop across the strainer as a function of time comprises:

(a) Said strainer,

(b) Means for sensing the pressure drop across said strainer,

Patented July 15, 1969 (c) Means for reversing said strainer with respect to the direction of stream flow or vice versa whenever the pressure drop across said strainer reaches a predetermined value, and

(d) Means for recording the number of reversals in a given time.

For a more detailed understanding of the invention reference is made to the following description and the attendant drawings wherein:

FIGURE 1 is a schematic diagram of an apparatus which may be used to determine pressure drop as a function of time; and

FIGURE 2 is a schematic diagram of an alternative apparatus which employs the same electrical circuitry as the apparatus of FIGURE 1.

Applicants have found that devices e.g., heat exchangers, filter beds, etc.) capabl eof removing solid material from process streams may be characterized as having an eitective number of openings, N of an effective diameter, d. By this characterization, the time for a device to develop a predetermined pressure drop, AP or a predetermined fiow rate, W may be determined by subjecting a strainer having N openings of diameter, d, to the process stream. The strainer may be either a perforated sheet or a screen having at least openings.

The effective diameter, d, is obtained by subjecting the device and various size strainers to a stream containing a substantial amount of material that does not pass through the device and selecting the size strainer which removes only such size material as would be entrapped by the device.

It has further been found that any apparatus, device or strainer, characterized as having openings of a given diameter, which removes solid material from a stream has a modus operandi defined by the following differential equation:

Since N/N equals VAP /AP, where AP and AP are the pressure drops across the apparatus at time zero and t, respectively,

N (1 /AP /AP) W-z If the pressure drop, AP, across the apparatus is constant, f is a constant. By integrating Equation a between the same limits as before,

Since N/N equals W/W where W is the total flow rate at time zero and f: W /N To illustrate the invention, the subsequent description will be limited to constant flow, incorporating only Equation b.

Both the aforementioned strainer and device are subjected to a test stream, containing a substantial amount of material that does not pass through either, for time periods t and t and at flow rates W and W respectively. The device and strainer may be placed in parallel or subjected to the test stream sequentially. The pressure drops across the device and the strainer at the start, AP and AP respectively, and the pressure drops after the time periods t and t AP and AP respectively, are measured.

By using Equation b for the strainer and the device, substituting the appropriate values and equating both, N the effective number of holes of the device may be determined by:

The pressure drop, AP is preferably predetermined either experimentally by calibrating clean screen flow rate v. pressure drop or empirically. The pressure drop, AP; is a function of the particular process depending upon a balance of factors, e.g., pump head, heat transfer, cost, yield, etc.

This determination, commonly coined the fouling cycle of the device, provides an important factor in considering the type and size of equipment employed, and the feasibility of periodic cleaning and/or installation of solids removal equipment. Thus, by characterizing a particular device as having N openings of diameter a, the fouling cycle may be determined without passing the process stream through the device.

By operating entirely at constant pressure drop across both the device and strainer, the time would be determined by the exclusive use of Equation c. A combination of constant flow rate and constant pressure drop conditions may be used. For example, the strainer could be operated at a constant flow rate and the device, at a constant pressure drop. By letting X1=1,2,3,4 equal either 1 /AP /AP at constant flow rate or in (W /W at constant pressure drop and equating Equations b and c the effective number of openings, N is generically represented by Also, if the steps in the above method are operated such that the flow rate otherwise has a fixed and determinable relationship to the pressure drop, Equation a may be appropriately integrated. This has general application where the process stream has neither a fixed pressure drop nor flow rate.

The apparatus shown in FIGURE 1 may be used with constant flow rate to obtain continuous values of AP vJ for Equation e.

The process stream enters from the left passing through constant flow orifice 1 (the How rate is W The stream then passes through the four-way motor valve 2. On one cycle the stream enters through port 3, exits through port 4, passes through strainer 5, re-enters through port 6 and finally exits through port 7 to drain. When the pressure drop across strainer 5, as sensed by differential pressure switch 8, reaches a given value, valve 2 is reversed. The flow path is then via ports 3 and 6, strainer 5, and ports 4 and 7. When the pressure drop reaches the same value again, valve 2 reverses and the original flow path is reestablished. For clarity valve 2 is illustrated with the ports lying in different planes. However, in prac tice, all ports extend radially in the same plane. This provides a minimum, equal pressure drop across the valve for both flow paths.

The following electrical circuit is only representative of a number of circuits which may be used in conjunction with the present invention.

Diaphragm 9 within switch 8 moves in response to the pressure drop between points 10 and 11. When the given pressure drop is reached, diaphragm 9 trips a microswitch closing contact 12 thereby allowing the 117 volt, 60 cycle supply current to pass into coil 13 of time delay relay 14.

This relay introduces a five second time lag between the time the current enters coil (13 and the time contact 15 closes. Therefore only a sustained signal will cause contact 15 to close. Otherwise a spurious line signal would activate the subsequent reversing relay 16. When contact 15 closes the signal is passed to coil 17 of reversing relay l6.

Counter 18. connected in parallel between time delay relay 14 and reversing relay .16, indicates the number of times contact 15 closes.

When the signal enters coil 17, contact 19 changes position. Thus, if contact 19 is in the lower position when contact 15 closes, it moves to the upper position until the next signal is received. When contact 19 contacts the upper position, four-way motor valve 2 is actuated allowing' the process stream to flow through one of the aforementioned paths. When contact 19 switches, the other path is open to flow.

If desired, each signal may be additionally recorded by providing a contact 20 which is mechanically linked to contact 19. By connecting in series, a power supply and a recorder, the signal can be recorded when contact 20 -isin the upper position.

Each time the stream flow is reversed, the entrained material on the surface of strainer 5 is backfiushed to drain. This allows each cycle to start at the same initial pressure drop, AP

An alternative apparatus is shown in FIGURE 2. The same electrical circuit as illustrated in FIGURE 1 is used. Instead of reversing the flow path, the strainer is reversed. Thus the entering process stream passes through constant flow orifice 21 and strainer 22 to drain. When the given pressure drop, as sensed by differential pressure switch 23, is reached, reversal motor 24 rotates the strainer the strainer is back flushed; and the reversal is recorded as before.

It will be understood that various changes in the details, materials, steps, order of steps, etc., which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention.

We claim:

1. A method for predetermining the time, 1 in which a device will develop a pressure drop, AP from a given starting pressure drop, A1 when subjected to a process stream flowing at a rate W the process stream containing a substantial amount of material that does not pass through said device, which comprises:

'(a) determining the equivalent number of openings of said device, N by subjecting to the same test stream said device and a strainer, said strainer having an opening size such that it removes only such size material as would be entrapped by said device, for time peri ds I, and r; and at flow rates W, and W re spectively, said strainer having N openings, N being at least 100, measuring the pressure drops across said device and said strainer at the start, AP and AP and the pressure drops after the time periods 2 and t AP and AP respectively, and determining N N being equal to AP 02 W, m) (b) thereafter, passing the process stream through the same strainer at a flow rate W for a time period t and measuring the pressure drop at the start,

AP and the pressure drop after the time period t AP and determining the time 1 said time being 2. The method of claim 1 wherein said device is a heat exchanger.

3. The method of claim 1 wherein said device is a filter bed,

4. The method of claim 1 wherein said strainer in said process stream is reversed whenever the pressure drop reaches a predetermined pressure drop AP 5. The method of claim 1 wherein the direction of flow of said process stream through said strainer is reversed whenever the pressure drop reaches a predetermined pressure drop AP 6. A method for predetermining the time, t in which the flow rate through a device falls to W from a given starting flow rate, W when said device is subjected to a process stream flowing therethrough, the process stream containing a substantial amount of material that would be entrapped by said device, the flow of said process stream being obtained by maintaining a constant pressure drop across said device, which comprises:

(a) determining the equivalent number of openings of said device, N by subjecting to the same test stream said device and a strainer, said strainer having an opening size such that it removes only such size material as would be entrapped by said device, for time periods 1 and t and at constant pressure drops across said device and said strainer, said strainer having N openings, N being at least 100, measuring the flow rates through said device and said strainer at the start, W and W 2, and the flow rates after time periods t and t W and W respectively, and determining N N being equal to Warn g (b) thereafter, passing the process stream through same strainer at a constant pressure drop across said strainer for a time period, t and measuring the flow rate at the start, W and the flow rate after the time period i W and determining the time, t.;, said time being equal to W N -ln (fig-W 1,

W W4N 1'11 7. A method for predetermining the time, L in which the flow rate through a device falls to W from a given starting flow rate, W when said device is subjected to a process stream flowing therethrough, the process stream containing a substantial amount of material that would be entrapped by said device, the flow of said process stream being obtained by maintaining a constant pressure drop across said device, which comprises:

(a) determining the equivalent number of openings of said device, N by subjecting to the same test stream said device and a strainer, said strainer having an opening size such that it removes only such size material as would be entrapped by said device, for time periods t and t and at flow rates W and W respectively, said strainer having N openings, N being at least 100, measuring the pressure drops across said device and said strainer at the start, AP and AP and the pressure drops after the time periods t and t AP and AP respectively, and determining N N being equal to (b) thereafter, passing the process stream through the same strainer at a constant pressure drop across said strainer for a time period, t;.;, and measuring the flow rate at the start, W and the flow rate after the time period t W and determining the time, 1 said time being equal to 8. A method for predetermining the time, i in which a device will develop a pressure drop, AP from a given starting pressure drop, AP when subjected to a process stream flowing at a rate W the process stream containing a substantial amount of material that would be entrapped by said device, which comprises:

(a) determining the equivalent number of openings of said device, N by subjecting to the same test stream said device and a strainer, said strainer having an opening size such that it removes only such size material as would be entrapped by said device, for time periods t and 1 at constant pressure drops across said device and said strainer, said strainer having N openings, N being at least measuring the flow rates through said device and said strainer at the start W and W respectively and the flow rates after time periods t and t W and W respectively, and determining N N being equal to W -t -1n (b) thereafter, passing the process stream through the same strainer at a flow rate W for a time period t and measuring the pressure drop at the start, AP and the pressure drop after the time period t AP and determining the time t,,, said time being equal to 9. An apparatus for continuously determining the time in which a strainer will develop a predetermined final pressure drop when subjected to a process stream containing a substantial amount of material that does not pass through said strainer, which comprises:

(a) said strainer,

(b) means for sensing the pressure drop across said strainer,

(c) means for reversing said strainer in said process 7 8 stream whenever the pressure drop across said strain- (d) means for recording the number of reversals in er reaches the predetermined final pressure drop, and a given time period whereby the time in which said (d) means for recording the number of reversals in strainer will develop a predetermined final pressure a given time period whereby the time in which said drop is determined. strainer will develop a predetermined final pressure 5 drop is determined. References Cited I 10.1163 apparatus for tlzfrginuiausly detegmtining tlge UNITED STATES PATENTS m w to a s ramer w1 eve 0p a pre e ermine na pressure drop when subjected to a process stream cong i 22} taining a substantial amount of material that does not 10 3138552 6/1964 a 5 pass through said strainer, which comprises: 1 31 11/1964 B s (a) said strainer, V ry on 210108 X (b) means for sensing the pressure drop across said LOUIS R PRINCE Primary Examiner strainer,

(c) means for reversing the direction of flow of said 15 JOSEPH W. ROS OS, Assistant Examiner process stream through said strainer whenever the pressure drop across said strainer reaches the prede- CL termined final pressure drop, and 73-38; 210-106 Egg? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. l, -5 5, M D t d July 15, 1969 FRANK T. SMITH and ROBERT D. SMITH It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

[- Column 5, line 55 the formula should read as follows:

1 ln W c m..- il-l Column 6, line 61 the formula should read as follows:

l.. 'l- P N 1- 9 L w 1; c P,

[V03 w n 1 t 1 i L. 3

SIGNED AND SEALED MAR24197U (SEAL) Attest:

Edward M. Fletcher, Jr.

WILLIAM E. SGHUYLER, JR. Attestmg; Offlfifi? Commissioner of Patents 

